On an Improved Construction of Ruhmkorff's Induction Coil, 519 



to have been communicated to the system by the enclosure, we 

 obtain 2748 units of heat, which would raise 200«f'"*45 of metallic 

 selenium from degree to the temperature •;. or(J iloirivr 



O74Q -- * rtf!tn 



'^^^^ -180°-15. .n 



200-45-0-0761 



This elevation of temperature is less than that which we have 

 found by the first mode of calculation ; but I shall show that it 

 is necessarily too little, for in the preceding calculation we have 

 assumed by inference, that at the moment at which the thermo- 

 meter marked the maximum in the oven, and at which it was 

 plunged into the calorimeter, the molecular transformation was 

 complete. But we know from the first experiment, that even 

 then the substance still continued to disengage heat, and we 

 allowed for that in our first calculation. 



In recapitulating, I think we may conclude from these expe- 

 riments that vitreous selenium disengages during its transform- 

 ation into metallic selenium a quantity of heat, which would 

 elevate the latter by more than 200 degrees. 



I formerly found [Ann. de Ch. et de Phys. 3rd series, vol. i. 

 p. 205) an analogous disengagement of heat during the trans- 

 formation of soft sulphur into ordinary sulphur, but the quan- 

 tity of heat liberated was much less, for it only raised the tem- 

 perature of the substance 12 to 14 degrees. It is true that the 

 sulphur is then brought almost to its fusing-point, so that part 

 of the heat disengaged may be disguised by partial fusion. 



LXVI. On an Improved Construction of Ruhmkorfi'^s Induction 

 Coil. By C. A. Bentley, Esq. 



To the Editors of the Philosophical Magazine and Journal. 



, 128 Sloane Street, Chelsea, 

 Gentlemen, Dee. 3, 1856. 



AS many of your readers may be interested in the production 

 of induction coils, I take the liberty of sending a short, 

 but, I trust, concise description of one I have constructed ; and 

 as it is the result of two years' experience, it will no doubt prove 

 valuable to those interested in electro-magnetic induction. 



The instrument referred to gives an induction spark 2 inches 

 long in air of ordinary density when excited by four or five cells 

 of Grove's battery; and the quantity of static electricity is so great, 

 that it will charge a quart Leyden jar two hundred times per 

 second, the discharge taking place through an interval of 1:| inch. 



The thermal phsenomena are also very striking ; for when the 

 secondary wires are separated about three-quarters of an inch, 



